In recent years, technology has been developed that gasifies organic solid material such as coal, biomass, and tire chips instead of oil in order to produce gasified gas. The gasified gas that is produced in this manner is used in efficient power generation systems known as Integrated coal Gasification Combined Cycle (IGCC) systems, and in the manufacturing of hydrogen, the manufacturing of synthetic fuel (i.e., synthetic oil), and in the manufacturing of chemical products such as chemical fertilizers (i.e., urea) and the like. Of the organic solid materials that form the raw materials for gasified gas, the ratio of reserves to production for coal, in particular, is approximately 150 years, which is approximately three or more times the ratio of reserves to production of oil. Moreover, because coal deposits are not unevenly distributed compared to oil, they are expected to be a natural resource that is capable of providing a stable supply for an extended period of time.
Conventionally, the process of gasifying coal is achieved by performing partial oxidation using oxygen and air, however, because an extremely high temperature of approximately 1800° C. and an extremely high pressure of approximately 3 MPa are required, special materials that are able to withstand high temperatures and high pressures are required so that the conventional technology has the drawback that the costs of the gasification furnace are extremely expensive. In order to solve this problem, technology has been developed that utilizes steam in order to gasify coal at lower temperatures of between approximately 700° C. to 900° C., and at normal pressure. This technology has the advantages that, by setting the temperature and pressure at lower levels, special structure to withstand high pressure is not required, and commercially available items already in common use can be employed.
However, in the above-described organic solid material steam gasification reaction, a comparatively long reaction time is required. Because of this, the gasification furnace main unit is designed having a size that ensures adequate residence time for the organic solid material to react sufficiently. In a gasification process of this type, it is necessary during actual use to adjust the gasification efficiency (i.e., the carbon conversion ratio). However, it is not realistic to change the residence time of the organic solid material by changing the volume of the gasification furnace main unit. Accordingly, as one method of indiscriminately adjusting the carbon conversion ratio in the gasification, a method in which the residence time of the organic solid material is changed by adjusting the amount of circulation of a fluid medium inside the gasification furnace main unit may be considered.
For example, it is possible to adjust the amount of circulation of a fluid medium very simply by adding the fluid medium to or extracting it from a gasification furnace. Moreover, technology has also been disclosed (see, for example, Patent document 1) in which a flow rate control unit adjusts the output quantity of the fluid medium that is output from the gasification furnace main unit in a circulating fluidized bed boiler apparatus. Furthermore, technology is also known (see, for example, Patent document 2) in which, by controlling the pressure inside a chamber that contains a fluidized bed in a circulating fluidized bed furnace, the bed height of the fluidized bed is adjusted, so that the amount of circulation of the fluid medium is controlled.